Transmissive spongy secondary emitter

ABSTRACT

An electrode includes an electron or irradiation transmissive conducting layer and an electron-emissive layer of insulating material in spongy form.  This layer may be of BaF2, LiF2, MgF2, MgO, Al2O2, CsI, KCl or NaCl and preferably has a density of only about 1% of the same material in bulk form, e.g. 0.01 to 0.1 gms. per cc. with a thickness of 10 to 100 m .  The layer may be formed by deposition in a gaseous atmosphere, e.g. argon at 1 to 2 mm. of Hg pressure, with a spacing of about 3 inches between the evaporator and the receiver, which may be rotating.  Alternatively magnesium may be burnt in air at atmospheric pressure about 14 inches from the receiver. The receiver may be an aluminium film supported by a metal ring and formed by vacuum deposition of aluminium on to a film of thermally removable cellulose nitrate to a thickness of 140 to 1000 .  Specifications 792,507, 862,211 and 898,433 are referred to.

July 27, 1965 R. J. SCHNEEBERGER 3,197,662

TRANSMISSIVE SPONGY SECONDARY EMIT'IER Filed March 11, 1960 Spongy Secondary Electron Emitting Loyer Source L Fig .l

v- 24 46 ii h Z :2 ii g! WITNESSES INVENTOR 15% Robert J. Schneeberger desirable in some applications.

United States Patent 3,197,662 TRANSMISSIVE SPONGY SECONDARY EMITTER Robert J. Schneeberger, Pittsburgh, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa, a corporation of Pennsylvania Filed Mar. Ill, 1964 Ser. No. 14,394 16 Claims. (Cl. 313104) This invention relates, generally, to electron discharge devices having components for the emission of electrons and, more particularly, to such components which emit secondary electrons upon primary electron bombardment. Such components are known in the art as secondary electron emitting electrodes or merely as dynodes. This invention also relates to such dynodes in combination with other tube components which supply the electrons incident to the dynodes or are responsive to the electrons emitted by the dynode. For purposes of this invention and the following description thereof, the term emission includes the passage of electrons from the emissive element into an immediately adjacent solid element as well as the situation in which the elements are spaced.

Much effort regarding secondary electron emission has been devoted to the problem of increasing the yield or secondary electron emission ratio which is the number of secondary electrons emitted per incident primary electron. The success of these eiforts is evidenced by the use of secondary electron emissive dynodes in electron multipliers for a variety of purposes. However, the demand persists for dynodes having even higher yields to enable improved results in ordinary electron multipliers and also to permit new applications.

Among the dynodes most commonly known to the prior art is that often called the front surface dynode. This type of dynode comprises a heavy conductive member upon which is deposited a layer of secondary electron emitting material. The conductive member is generally maintained at a potential more positive than the source of primary electrons but more negative than the secondary electron collector. In such structures primary electrons impinge upon the exposed surface of the secondary electron emitting material and secondary electrons are emitted from the same surface. Such devices have an obvious inherent degree of geometrical inflexibility which is un- Efforts to alleviate this problem resulted in dynodes which became known in the. art as venetian blind dynodes and open mesh-type dynodes. These components also emit secondary electrons from the same surface upon which the primary electrons impinge but, because of the openness of the structure, the secondary electrons can be reversed in course by the application of a strong enough electric field. A well-known application of a multiplier of this type is that employed in commercially available pick-up tubes of the image orthicon type.

A significant advance in dynode design was achieved with what are now termed transmission secondary electron emitting dynodes or merely transmissive dynodes. Such dynodes are described in US. Patent 2,905,844 by E. J. Sternglass entitled, Electron Discharge Device, issued September 22, 1959, and assigned to the same as-' signee as the present invention. Transmissive dynodes comprise a layer of secondary electron emitting material which is impinged on one surface by primary electrons producing secondary electrons which are emitted from the opposing surface. Such dynodes have a variety of advantages and additional possible applications over those in which front surface dynodes may be employed, as is pointed out in the referred-to patent. One particular field of application for this type of dynode is in image 'ice intensifier devices as disclosed in the above patent and in US. Patent 3,114,044 entitled, Electron Tube, by

E. l. Sternglass, issued December 10, 1963, and assigned to the same assignee as the present invention. A principal advantage of such devices in imaging is that image resolution may be preserved as the electron image traverses the space between the photocathode and the first dynode or between subsequent dynodes. Despite removal of geometrical restrictions by the transmissive dynode in the field of image intensification, it has still been difiicult to obtain images of sufficient brightness intensification. Also, difliculties in fabrication have been encountered because of the crystalline nature of the secondary electron emissive material which makes it prone to crack upon thermal cycling during tube processing and exhibit other forms of instability such as crystal growth and crystal reorientation.

It is, therefore, an object of this invention to provide electron discharge devices having improved electron emissive elements.

It is another object of this invention to provide a secondary electron emitting element which has a high yield.

It is another object of this invention to provide a secondary electron emitting element having a high secondary electron yield and which, furthermore, is of the transmissive type.

Another object is to provide an improved dynode structure which may be readily made at low expense.

Another object is to provide a dynode having a secondary electron emitting layer which is physically and chemically stable under conditions including thermal cycling and electron bombardment.

Another object is to provide an improved dynode structure which has particular applicability in image intensifier devices.

Another object is to provide multi-layer tube components which include a transmissive secondary electron emitting layer having high yield.

Another object is to provide a method of making a secondary electron emitting element having improved yield and stability.

According to the present invention, an electron emitting element is provided which includes a member of a secondary electron emitting material in a spongy form. The spongy secondary electron emitting material has a density which is very low compared to the bulk density of the same material. According to a further feature of the invention, such a member is provided as a transmissive type dynode. According to further features of. the invention such a member is provided in combination with other radiation or electron sensitive materials to provide a variety of devices.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The present invention, both as to its organization and manner of operation, together with the above as well as further objects and advantages thereof, may best be understood by reference to the following description, taken in conjunction with the accompanying drawings, in which:

FIGURE 1 is a partial, sectional view of a secondary electron emitting component in accordance with the present invention;

FIG. 2 is a partial, sectional view of a modification of the dynode of FIG. 1;

FIGS. 3 through 7 are partial, sectional views of secondary electron emitting components according to the present invention in conjunction with other elements of a radiation sensitive or electron sensitive nature; and

FIG. 8 is a sectional view of an image intensifier device having a transmission secondary electron multia negligible number plier employing dynodes in accordance with the present invention.

Referring now to FIG. 1, there is shown a dynode 10 comprising a support ring 11 having a support layer 12 of material stretched across it. Upon the support layer 12 there is disposed a layer 14 of secondary electron emitting material in spongy form in accordance with the sen invention. ini electron source 13 is shown to the left of the dynode 10 emitting electrons directed thereto under the influence of an electric field produced by a su table potential source (not shown). Any electron emitter may be employed as the source 13 including point sources, planar sources, cold cathodes, thermionic cathodes and photocathodes. While not shown in FIGS. 2-5, an electron source 13 is, of course, also used therewith.

The emissive layer 1 comprises an insulator material or materials'which are generally characterized in having a relatively large electron energy band gap. The band gap refers to the minimum energy necessary to excite an electron from the filled, or valence, band of the material into the conduction band. The materials of which the spongy layer 14 is comprised have a band gap of about 3 electron volts or more. It has been found that most successful results in accordance with the present invention are achieved when employing insulator materials in the range from about electron volts to about electron volts, however, the upper limit 15 imposed by the lack of materials having larger band gaps winch may be readily fabricated rather than by any inherent limitation. V

Among the materials suitable for use in accordance with the present invention are barium fluoride, BaF lithium fluoride, LiF magnesium fluoride, MgF g magnesium oxide, MgO; aluminum oxide, A1 0 cesium iodide, Csl; potassium chloride, KCl; and sodium chloride, NaCl. In accordance with a' preferred embodiment of the present invention, the emissive layer 14 consists essentially of barium fluoride because of th s mate r zus combination of good electrical characteristics, staoihty under thermal cycling and electron bombardment, and ease of fabrication.

The secondary emissive layer 14 1s one or more of the materials just previously discussed in spongy form. The spongy layer 14 is of a soft consistency and full of cavities or pores and may be said to have an open, loose or pliable texture. The sponginess of the layer 14 1s a result of the fact that the material therein exists in a condition of fine division and loose coherence. In gross appearance, the spongy layer 14 resembles a deposit of a very fine soot or dust, and is referred to some workers in the field as a smoke layer or a smokedeposited layer for reasons which Wlll become more apparent in the discussion hereinafter.

Matter in spongy form and particularly the material used in the spongy emissive layer 14 has a volume density which is much less than the volume density of the same material in the bulk state. Material in the bulk state, as opposed to material in spongy form, is generally of a crystalline nature comprising large crystals having a definite orientation of molecules therein, such crystals being very closely packed together so that there are only of voids therebetween comprising very little volume. Solid materials are ordinarily considered to have a fairly definite density. This density is termed the bulk density of the material since it is determined by considering only the volume of matter and ignoring the voids'therein. Compared with the bulk density of the same material or materials, the spongy emissive layer 14 has a density which is only about 2 percent or less of the bulk density. It is preferable that the density be of about 1 percent of bulk density. Layers having such a density have been found to provide a high yield.

The spongy emissive layer 14, because of its low density compared to the same materials in the bulk state, has a volume density of from about 0.01 gram per cubic centimeter to about 0.1 gram per cubic centimeter. It is usually more convenient to consider the spongy emissive layer 14 in terms of its area density which may vary in the range from about micrograms per square centimeter to about 200 micrograms per square centimeter. The thickness of the spongy emissive layer generally falls within the range from about 10 microns to about microns.

The spongy layer 14 of secondary electron emissive material is found upon close examination by optical or electron microscope to comprise individual particles in larger clusters. The individual particles are believed to be small aggregates of many tiny crystals rather, than of a single crystal. They have a size which ranges from about 160 angstroms to about 500 angstroms in diameter. The larger clusters which do not necessarily have a spherical shape but may be quite irregular or of a flakey shape, have a diameter of the order of about 1 to 10 microns. They comprise a plurality of the small particles loosely joined together.

A spongy secondary electron emissive layer 14 is formed by deposition onto the dynode support means in a gaseous atmosphere. it has been found that material deposited in a vacuum has a crystalline nature and in other respects as well is not different from the ordinary solid material. As gas pressure is increased it is found that the deposited layer has less density and generally more of the characteristics of a spongy layer in accordance with this invention. Hence the layer attains the characteristics of a deposit from a smoke. Generally, the greater the gas pressure is, the less dense the deposited layer will be. It has been found that a layer having a density of the order of 1 percent of the bulk density of the material is formed when deposited in a gaseous atmosphere of a pressure of about 1 or 2 millimeters of mercury and a spacing between the evaporator and receiver of about 3 inches. A more dense layer ordinarily results when the evaporator to receiver spacing is decreased. However, the pressure may e increased to compensate for this. The gas employed in the deposition process is generally selected for its inertness to the material to be deposited. However, it may be desirable to use a gas which combines with evaporated material to form a layer of the reaction product, if such reaction product is of the desired nature.

Generally, the emissive material is placed in a suitable boat provided with a resistance heating element. The material at this stage is in solid or chunk form. At a suitable distance from the boat is placed the dynode support means or other receiver for the vaporized material. To insure a uniform deposition, it may be desirable to employ a rotating receiver. Such a device also makes it possible to deposit several emissive layers in one operation.

The support means 1% shown in FIG. 1 is such that it permits operation of the spongy emissive layer 14 as a transmissive secondary electron emissive layer. The support ring 11 therefore supports the support layer 12 and the spongy emissive layer 14 only at their periphery. The support layer 12 is generally necessary because of the spongy character of the emissive layer 14 which makes it practically incapable of self-support. The support layer 12 should be of such a material and thickness that it'is penetrable by primary electrons incident thereon.

The support layer 12 may, as a specific example, be a thin self-supporting aluminum film. An aluminum film may be formed by the vacuum deposition of aluminum onto a film of thermally removable organic material such as cellulose nitrate. A thickness of aluminum of from about angstroms to about 1000 angstroms is suitable. A thin layer is preferred so that the voltages by which the primary electrons are accelerated need not be excessive for penetration through the support layer 12 into the spongy emissive layer 14. The support layer may, alternatively, be of aluminum oxide formed in accordance with the teachings of US. Patent 2,898,499 by E. J. Sternglass and W. A. Feibelman entitled, Transmission Secondary Emission Dynode Structure, issued August 4, 1959, and assigned to the same assignee as the present invention. It is desirable, however, that the support layer 12 have sufficient conductivity so that electrons emitted from the dynode may be replaced. Such an expedient therefore prevents the emissive layer 14 from charging up after continued operation which would substantially reduce the number of secondary electrons emitted. Adequate conductivity may be supplied by a conductive grid support. However, as beforestated, the spongy character of the layer 14 generally makes a continuous support layer necessary.

A specific example of a dynode structure in accordance with the present invention and the method of forming such a structure will now be described. An aluminum foil was formed across a support ring in the manner described above. The diameter of the aluminum support layer, which had a circular shape was about 0.75 inch. Such a support member was placed in a bell jar having an atmosphere of approximately 1 millimeter mercury of argon gas. Also in the bell jar was disposed a tantalum boat having a resistive heating element therein and containing about 16 milligrams of barium fluoride in solid or chunk form. The boat was placed a distance of approximately 3 inches below the dynode support. Current was applied to the resistive heating element and heating con tinued until it was observed that the barium fluoride had just melted, at which temperature the material was then maintained. The material was therefore at its melting point under the conditions then obtaining, namely, a gas pressure of about 1 millimeter of mercury. Therefore this temperature is considerably less than the melting point at atmospheric pressure which is about 1280 C. The vapor pressure of barium fluoride at its melting point under such conditions is found sufficient to cause vaporization of the material at a sufficient rate. The barium fluoride was evaporated to completion and it was found that the area density of the evaporated material on the dynode support was approximately 87 micrograms per square centimeter. Such layer had a thickness of approximately 20 microns, Therefore it is seen that while barium fluoride has a bulk density of about 4.838 grams per cubic centimeter, a spongy layer formed in the man ner just described has a density of the order of only about 6.04 gram per cubic centimeter.

An additional manner in which the dynode of FIG. '1 may be formed will now be described. A quantity of magnesium metal, in the form of a thin ribbon, was placed at the lower end of a line of about 14 inches length. A dynode support structure, comprising an aluminum film, as was previously described, was placed at the upper end of the flue. This arrangement was placed in air at atmospheric pressure. The magnesium was ignited, forming MgO which was propelled by the time draft to the receiver. A spongy layer having an area density of about 70 micrograms per square centimeter was formed.

In operation, primary electrons 50 from the source 13 bombard the support layer 12 of the dynode to which they may be accelerated by the application of a suitable electric field. The primary electrons penetrate the support layer 12 but may be deflected somewhat from their original course in so doing. The primaries 5% then bombard the spongy secondary electron emitting layer 14. Copious electron emission 66 from the surface 15 of the spongy layer 14 remote from the electron source of from about 5 up to about secondaries per primary has been observed. The primary electrons 54 should have sufiicient energy to penetrate the support layer 12 and to make several collisions in the spongy emissive layer 14. Therefore, energies of several times the band gap of the emissive material of the layer 14- are needed. For efficient utilization of the primaries in the production of fine particles conform readily to any thermally imposed 14 will be for a givenapplied potential.

6 secondaries, it is also desirable for the primaries to lose substantially all their energy due to collisions prior to reaching the emissive surface 15 of the layer 14. Most primaries are therefore absorbed within the layer 14 and only secondaries 69 are emitted therefrom. However, some primaries may penetrate entirely through both layers 12 and 14- but the number of these primaries is only about 10 percent or less of the total impinging on the device.

It is believed that, in contrast to what occurs in crystalline materials, electrons are able to traverse a relatively long path in the spongy emissive layer 14 because of the voids therein. Secondary electrons emitted within the material are able to travel through the pores and excite additional secondaries in what may be an avalanche effect. It is believed that upon initial emission a positive charge will remain upon the surface 15 of the spongy secondary electron emitting layer 14 which establishes an internal electric field within the spongy emissive layer 14 which further accelerates electrons and enables additional secondary emission.

To further enhance this field dependent effect an additional conductor 16 may be provided external to the dynode to which an accelerating potential may be applied to further increase the field. Such a conductor 16 while not shown in the remaining figures may be employed therewith if found desirable. A suitable potential source 17 may be employed connected to the conductive mesh 16 and the support layer 12 of the dynode, which should be conductive for this purpose, by means of the leads 18 and 19, respectively. The greater distance the conductive mesh 16 is spaced from the emissive layer 14, the less the field across the emissive layer Therefore it is generally desirable that the conductive mesh 16 be placed relatively close to or in contact with the emissive layer 14. Where continuous emission is desired the potential supplied by the source 17 would be a direct current potential. However, if desired for particular applications a pulsed D.C. source or an AC. source could also be employed. In addition, a holding electron gun may be employed to hold the surface 15 of the emissive layer 14 at a uniform potential in the manner in which a gains in excess of 10 secondaries per incident primary.

Besides enhance-d yields, secondary electron emitting dynodes in accordance with this invention provide additional advantages over those comprising vacuum-deposited crystalline materials. For example, because of the necessity to heat cycle the dynode structure during tube fabrication and processing, differences in the coefficients of expansion of the emissive layer and the support members, crystal growth and crystal reorientation in the secondary electron emitting material cause crystalline dynodes to break with resulting non-uniformity in secondary emitting properties. Utilization of a spongy layer in accordancewith this inventionavoids this problem because the geometrical configuration. Furthermore, heating caused in operation by application of the potentials to the support elements and electron bombardment may have a similar deleterious effect with crystalline materials which is avoided by use of the spongy emissive layer 14 in accordance with this invention. 7

The spongy secondary electron emitting layer 14 shown in FIG. 1 may be employed in various configurations of which those of the other figures in this applications are representative.

FIG. 2 employs a support means of members it and 12, substantially as shown in FIG. 1, upon which a layer of a crystalline insulating material formed by deposition in a vacuum is disposed. A layer 14 of spongy secondary electron emitting material is disposed on the crystalline layer 20 remote from the support layer 12. The crystalline layer 20 may be of a material selected from the group from which the material for the layer 14 is selected. The crystalline layer would be of a thickness of only about 300 angstroms. It may be desirable to use the dynode of FIG. 2 which includes the crystalline layer 20 because it provides additional electrons which in turn bombard the spongy layer 14.

FIG. 3 shows a modification wherein a spongy secondary electron emitting layer 14 is disposed adjacent a layer 22 of material exhibiting the property of electron bombardment induced conductivity. An example of a material which may be used for the latter layer is selenium. Secondary electrons emitted from the emissive layer 14 alter the conductivity of the adjacent layer 22 and make it possible to derive an electrical signal therefrom which may sequentially represent the elements of an image, for example. It is, of course, obvious that the electron bombardment induced conductivity layer 22. may be spaced from the spongy layer 14. The configuration of FIG. 3 makes. clear that the emissive surface 15 of the spongy layer 14 need not be a free, exposed surface. The layer 22 may be deposited directly on the spongy layer 14 by techniques well known in the art.

FIG. 4 shows a modification in which an electron sensitive phosphor screen 24 is disposed adjacent the emissive layer 14 to provide an optical image of the electrons emitted. Of course, here too the phosphor layer 24 may be spaced from the emissive layer 14 rather than immediately adjacent it.

FIG. 5 shows two dynodes 26 and 27 employed in a single device to obtain an enhanced yield. Of course, the number of dynodes used need not be limited to two. The resulting output means 28 may comprise a phosphor screen 24 such as discussed in connection with FIG. 4 or a layer 22' of electron bombardment-induced conductivity material as discussed in connection with FIG. 3. A suitable potential source 30 provides electric fields between the dynodes 26 and 27 and between the last dynode 27 and the output means 28 in order that electrons emittedby the first or second dynodes 26 and 27 are accelerated to the next electrode. The potential source 30 therefore is not primarily to provide the function of a potential in an arrangement such as that shown in FIG. 5, conductive meshes may be employed relatively close to or in contact with the emissive side of the dynode to provide field enhanced secondary emission.

FIG. 6 shows a light sensitive device comprising an input window 32 having a transparent conductive layer 34 thereon and then a layer 14' of spongy secondary electron emitting material within which there are mixed particles 36 of a photoelectrically emissive material such as cesium antimonide. Initial electron emission is triggered by light striking the photoemissive material 36. Subsequently, secondary electrons are formed in the emissive material of layer 14. The spongy layer 14' may be formed having the photoemissive particles 36 therein by including the material 36 in the material to be deposited in accordance with the methods previously discussed.

FIG. 7 shows a modification of the structure of FIG. 6 wherein a separate photoemissive layer 36 is provided 8 next to the transparent conductor 34 for the emission of primary photoelectrons into the secondary electron emitting layer 14.

The embodiments of FIGS. 6 and 7 could both be displaced by a structure wherein the spongy emissive layer 14 itself emits electrons upon excitation by radiation. In such case, no photoemissive material 36 would be required.

FIG. 8 shows an image intensifier device wherein dynodes in accordance with this invention are particularly applicable and advantageous. A photoemissive cathode 36, a plurality of dynodes 40 and a phosphor output screen 24 are disposed in parallel arrangement. The dynodes 40 comprise a support structure and spongy secondary electron emissive layer as was discussed in connection with FIG. 1. A structure similar to that shown in FIG. 2 may also be employed. The spaces between the electrodes 36, 40 and 24 are effectively compartmentalized in the manner discussed in the before-mentioned U.S. Patent 3,114,044 so that light may not travel through out the tube and, also so that contaminating material from the photocathode 36 cannot reach the emissive surfaces of the dynodes 40. The dynode supports 42 extend through and are sealed to insulating spacers 44- of a suitable ceramic material which comprise the vacuum envelope of the device. A suitable optical system 46 is employed to focus upon the photocathode 36 an image to be intensified. Photoelectrons emitted therefrom are focused by a homogeneous magnetic field, which may be provided by the winding 48, upon the first dynode stage 41 where additional electrons are emitted. Subsequent dynodes are provided for additional amplification. It is found that such. a device made in accordance with the present invention provides a gain of about five or more per dynode stage when a voltage of from about 3500 volts to about 4000 volts is provided between dynodes by a potential source which may be like the source 30 of FIG. 5. This multiplication occurs at each stage. It is seen that the total yield of a five-stage device may therefore be of the order of 1000. Higher voltages between stages would provide higher gains or enable a reduction in the number of stages. A grid 49 may be employed to act as a shutter controlling electron flow. Further details of the device of FIG. 8 including additional advantages there are discussed in the before-mentioned U.S. Patent 3,114,044.

While the present invention has been shown in a few forms only, it will be obvious to those skilled in the art that it is not so limited but is susceptible of various changes and modifications without departing from the spirit and scope thereof.

I claim as my invention:

1. A secondary electron source comprising a layer of insulator material on a conductive and electron penetrable support member, said layer of insulator material consisting of matter in spongy form having a density of less than 2 percent of the density of said insulator material in the bulk form.

2. A source of secondary electrons comprising a conductive and electron penetrable support member, a layer of insulator material on said conductive support member, said layer of insulator material comprising matter in spongy form having a density of substantially 1 percent of the density of said insulator material in the bulk form.

3. A source of secondary electrons comprising a conductive and electron penetrable supportmember, a layer of insulator material on said conductive support member, said layer of insulator material comprising matter in spongy form'having an area density of from substantially 50 micrograms per square centimeter to substantially 200 micrograms per square centimeter and a thickness of from substantially 10 microns to substantially microns.

4. A secondary electron source comprising a conductive and electron penetrable support member, a layer of insulator material on said conductive support member, said layer of insulator material comprising matter in spongy II F 8,191,

form having a volume density of from substantially 0.01 gram per cubic centimeter to substantially 0.1 gram per cubic centimeter.

5. A secondary electron source comprising an electron penetrable support member, a layer of insulator material comprising at least one member of the group consisting of barium fluoride, lithium fluoride, potassium chloride, sodium chloride, cesium iodide, aluminum oxide, magnesium oxide and magnesium fluoride on said conductive support member, said layer of insulator material consisting of matter in spongy form.

6. A secondary electron source comprising a conductive support member, a layer of insulator material comprising at least one member of the group consisting of barium fluoride, lithium fluoride, potassium chloride, sodium chloride, cesium iodide, aluminum oxide, magnesium oxide and magnesium fluoride on said conductive support member, said layer of insulator material comprising matter in spongy form having a density of less than 2 percent of the density of said insulator material in the bulk form.

7. A secondary electron source comprising a conductive support member, a layer of insulator material comprising at least one member of the group consisting of barium fluoride, lithium fluoride, potassium chloride, sodium chloride, cesium iodide, aluminum oxide, magnesium oxide and magnesium fluoride on said conductive support member, said layer of insulator material comprising matter in spongy form having a volume density of from substantially 0.01 gram per cubic centimeter to 0.1 gram per cubic centimeter.

8. A secondary electron source comprising a conductive support member, a layer of insulator material consisting essentially of barium fluoride disposed on said conductive support member, said layer of insulator material comprising matter in spongy form.

9. A source of secondary electrons comprising a conductive support member having thereon a layer of insulator material consisting essentially of barium fluoride, said layer of insulator material comprising matter in spongy form having a density of less than 2 percent of the density of said insulator material in the bulk form.

10. A secondary electron emitting element comprising a conductive support member, a layer of crystalline insulator material and a layer of insulator material in spongy form disposed on said crystalline insulator material remote from said conductive member, a source of electrons facing said conductive member and emissive of electrons penetrating therethrough.

11. In an electron discharge device, the combination comprising a conductive support member exposed to a source of electrons, a layer of insulator material in spongy form disposed on said conductive support member remote from said electron source and emissive of electrons in the same direction as electrons from said electron source, and a layer of material exhibiting the property of electron bombardment induced conductivity disposed in contact with said layer of insulator material to receive electrons emitted therefrom.

12. In an electron discharge device, the combination comprising a radiation transmissive window, a radiation transmissive conductive layer disposed on said window and a secondary electron emissive layer having photoemissive material dispersed therein disposed on said conductive layer, said secondary electron emissive layer comprising an insulator material in spongy form.

13. In an electron discharge device, the combination comprising a radiation transmissive window, a radiation transmissive conductive layer disposed on said window, a photoemissive layer disposed on said conductive layer and a layer of secondary electron emissive material disposed in contact with said photoemissive layer, said secondary electron emissive layer comprising an insulator material in spongy form.

14. An electron discharge device comprising at least one secondary electron emitting element including a layer of insulator material in spongy form emissive of secondary electrons from one surface when bombarded by primary electrons on the other surface and means to support said layer of insulator material on the surface thereof to which primary electrons are incident and to replace electrons emitted from said layer of insulator material.

15. An electron discharge device comprising at lease one secondary electron emitting element including a layer of secondary electron emissive insulator material in spongy form having a density of substantially 1 percent of the density of said insulator material in the bulk form, said layer of insulator'material being emissive of secondary electrons from one surface when bombarded by primary electrons on the other surface and conductive means associated with said layer of insulator material to replace electrons emitted therefrom.

16. In an electron discharge device, the combination including a plurality of secondary electron emissive dynodes each comprising a layer of secondary electron emitting material of at least one member of the group consisting of barium fluoride, lithium fluoride, potassium chloride, sodium chloride, cesium iodide, aluminum oxide, magnesium oxide and magnesium fluoride in.

spongy form having a density of substantially 1 percent of the density of said insulator material in bulk form, said dynodes being disposed in parallel relationship so that upon the incidence of an electron image to a surface of a first of said dynodes the image is reproduced in amplified form by secondary electrons emitted from the other surface of said dynode so that it may be applied to the next adjacent dynode, each of said dynodes comprising a conductive member disposed on the surface thereof to which primary electrons are incident serving as a means to replace electrons emitted from said dynodes.

References Cited by the Examiner UNITED STATES PATENTS 2,443,547 6/48 Weimer 131-68 2,898,499 8/59 Sternglass 313l03 2,905,844 9/59 Sternglass 3l3--67 2,910,602 10/59 Lubszynski 313 GEORGE N. WESTBY, Primary Examiner.

RALPH G. NILSON, ARTHUR GAUSS, Examiners. 

1. A SECONDARY ELECTRON SOURCE COMPRISING A LAYER OF INSULATOR MATERIAL ON A CONDUCTIVE AND ELECTRON PENETRABLE SUPPORT MEMBER, SAID LAYER OF INSULATOR MATERIAL CONSISTING OF MATTER IN SPONGY FORM HAVING A DENSITY OF LESS THAN 2 PERCENT OF THE DENSITY OF SAID INSULATOR MATERIAL IN THE BULD FORM. 